1. Field of the Invention
This invention relates in general to earth-boring bits and more particularly to the design of cutters for earth-boring bits for boring relatively large-diameter holes in mining and civil construction applications.
2. Description of Related Art
Earth-penetrating tools are divided generally into two broad categories, those designed to drill deep, relatively small-diameter boreholes and those designed to drill shallow, large-diameter boreholes. Earth-boring bits with rolling cutters mounted on cantilevered bearing shafts often are called xe2x80x9crock bitsxe2x80x9d and are employed in drilling relatively small-diameter boreholes for the recovery of petroleum or hydrocarbons, mining minerals, or to tap geothermal energy sources.
Another type of earth-boring bit or head employs a plurality of rolling cutters, usually in excess of three, arranged to drill relatively large-diameter boreholes for mining, tunneling, or other civil construction applications. In mining or boring operations, the bit or head is secured to a drilling machine and is rotated and pushed or pulled through formation material to bore a shaft or tunnel. The cutters of these bits generally are divided into two broad categories: those that rely on protruding hard metal, usually tungsten carbide buttons or inserts, to fracture formation material, and those that rely on raised discs to fracture the formation. The cutter assemblies employing tungsten carbide buttons or inserts generate high contact or point loads at generally very small areas in the formation, resulting in relatively small cuttings and fine, abrasive particles of rock. Conversely, the disc cutter assemblies employing rings scribe circles around the formation material to be disintegrated, resulting in spalling of large cuttings or pieces of formation material. The relatively large cuttings resulting from the action of disc cutter assemblies are regarded as preferable to the smaller cuttings generated by the button or insert cutter assemblies because they require less energy-per-volume of rock removed to fracture and are easier to remove from the borehole.
There are generally two types of disc cutter assemblies. In one type, the rings or discs are formed integrally with the cutter shell material and, when worn, necessitate replacement of the entire cutter shell or sleeve. In another type, the rings are annular kerf rings replaceably secured to the cutter shell or sleeve and can be removed and replaced easily when worn. The kerf rings of the latter type of cutter assembly generally are formed of unreinforced steel or are provided with protruding hard metal inserts to take advantage of both of the fracture modes discussed above. Those formed of unreinforced steel wear too quickly in abrasive rock formations, necessitating frequent replacement. Those kerf rings with excessively protruding inserts tend to operate in a fracture mode more similar to the cutters employing solely hard metal inserts or buttons as the cutting structure, rather than in the more advantageous disc cutter mode.
The advantages of both a disc cutter and reinforced steel have been obtained by constructing replaceable kerf rings having a single kerf with tungsten carbide inserts embedded in the kerf. These kerf rings allow removal of large portions of formation and provide superior wear resistance to unreinforced steel kerfs.
When mounted on a cutter, a singular kerf ring gives a spacing across the rock formation at 3 inches or more. Singular kerf rings with tungsten carbide insets have been used with some success, but as the spacing in between kerfs opens up to 2.5 inches or more in hard competent rock, the tungsten carbide inserts cannot handle the spacing or tight turning radius of the large diameter rings.
A need exists, therefore, for a cutter assembly for an earth-boring bit or head that employs the advantageous fracture mode of disc cutters with metal inserts, but that reduces the spacing between kerfs on a cutter.
It is a general object of the present invention to provide an improved disc-type earth-boring bit or head for mining or civil construction applications. This and other objects of the present invention are achieved by providing a generally circular bit body. A plurality of saddle members are secured to the bit body to receive and support each end of a plurality of corresponding journal members. A cutter shell or sleeve is mounted for rotation on bearings on each journal member.
A kerf ring having at least two kerfs is releasably attached to the cutter shell. Each kerf includes a longitudinal axis and a pair of opposing sides that converge to define a crest for disintegration of formation material. The kerfs are oriented such that the longitudinal axis of each kerf is inclined relative to a radial axis that bisects the kerf ring. The two kerfs diverge as they extend radially outward from the kerf ring. A plurality of hard metal inserts are embedded and secured in rows to each kerf, the inserts being generally flush with the sides and extending to the crests of the kerfs.
According to a preferred embodiment of the present invention, the hard metal inserts in one of the kerfs are axially aligned with spaces between inserts in the other kerf. This positioning of the hard metal inserts allows for the placement of the maximum count of hard metal inserts in the two kerfs without affecting the integrity of the kerf ring.
Other objects, features, and advantages of the present invention will become apparent with reference to the drawings and detailed description which follow.